You need to show some work before we can help you. (This really belongs in the homework section too). You are correct that you need to differentiate and use the chain rule. Perhaps you should have a go and post what you get.

Ok, so either you're not coming back, or you still can't attempt the problem. I'll give you a few hints: You vasically have two parametric functions a(y)=L(1-cos(y)) and t(y)=M(y-sin(y)), for some constants L and M. In the Friedmann equation, we have a term [itex]a'=\frac{da}{dt}[/itex]. Using the chain rule we can write [tex]\frac{da}{dt}=\frac{da/dy}{dt/dy}[/tex] from which you should be able to solve.